1. Field of the Invention
The present invention relates to non-impact printing apparatus and methods for recording using light-emitting diodes or the like.
2. Brief Description of the Prior Art
In the prior art as exemplified by U.S. Pat. No. 4,831,395 printing apparatus is described which comprises a multiplicity of individually addressable and energizable point-like radiation sources, such as LED's arranged in rows for exposing points upon a photoreceptor during movement thereof relative to and in a direction normal to the rows. Driver circuits are provided for simultaneously energizing the radiation sources responsive to respective data bit input signals applied to the driver circuits during an information line period. The print or recording head includes a support upon which are mounted chips placed end to end and upon each of which are located a group of LED's. The driver circuits are incorporated in chips and located to each side of the linear array of LED chips. The driver circuits in this apparatus include a shift register for serially reading-in data-bit signals and for driving respective LED's in accordance with the data signals.
While the printing apparatus described in the aforementioned patent works well, there are limitations inherent to a binary printhead recording using one bit per picture element (pixel), i.e., an LED is either on or off for recording of an image pixel. It is also known that improved image recording obtains when recording using multibits per pixel, i.e., grey level recording of each pixel. In this regard, U.S. Pat. No. 4,074,319 describes a printhead wherein a shift register stores for each pixel to be recorded a multibit signal for recording a grey level pixel. The respective data bits are shifted to each LED in response to a shifting clock signal which may be linear or binary weighted. A problem with printheads of this type is that it may require additional data lines to the printhead which take up valuable space thereon and require additional labor for forming wire bonds on the printhead to connect these extra lines to the driver chips. Alternatively, the data may be shifted in serially until all registers are full but doing so reduces recording rates and hence productivity, since printing of a line of pixels is limited to at least the time for filling all the registers on the printhead. It is noted in U.S. Pat. No. 4,074,319 that only one bit per pixel need be stored in the shift registers on the printhead with the remaining bits stored in RAM memory. With this approach, less room is required on the printhead for data lines and less labor required for construction of the printhead. However, the use of a RAM for storage of the data requires relatively complicated circuitry for formatting the data to the printhead and for providing addressing signals to the RAM to extract the appropriate data in the proper order.
A further disadvantage of the above prior art is that recording speeds are dependent upon the exposure duration for recording grey level pixels.
In order to increase productivity, it has been proposed in U.S. Pat. No. 4,679,057 that a laser beam recording apparatus be provided with pulse-modulating input image signals wherein both amplitude and pulsewidth modulation of the image-recording beam occurs. Where multiple recording elements are present as in an LED printhead and require simultaneous energization, the problem of efficiently providing data to the printhead arises. Efficient data handling may be expressed in terms of simplicity of the printhead architecture as well as the data rates required for transmitting data to the printhead.
It is therefore an object of the invention to provide an improved printer apparatus and method of recording providing a grey level printing capability and employing a plurality of recording elements each of which is energized to produce a recording beam that is modulated in both amplitude (intensity) and duration for each recording of a picture element (pixel).